Implementation of full color information systems for the display of high definition video information and complex pictorial and graphics images must be able to provide (1) high image resolution for precise edge definition and image sharpness; (2) high display and image luminance for maximum image brightness in a variety of display environments; and (3) precise, predictable control over color synthesis and reproduction using the largest color gamut available. Full color, high information content display systems must also be (4) small and compact, and (5) must be able to be manufactured at a low unit cost in order to be useful in a wide variety of applications. Existing color displays using a variety of technologies to produce full color, high resolution displays are deficient in one or more of these enumerated goals. For purposes of the discussion herein, the term "full color" display means a display which is capable of producing color from the full spectrum of visible light, and which uses at least three additive or subtractive primary colors to produce the full spectrum.
The dominant color production system used for the production of direct-view full color visual displays is an additive color system based on spatial juxtaposition, or spatial proximity, in which a single, full color picture element, or "pixel", of a displayed image is produced by the spatial integration of very small, juxtaposed primary (red, green, and blue) color sub-picture elements, or subpixels. "Pixel" and "image pixel" are defined herein as the smallest information element in a displayed image. The resolution of an image is determined by its pixel density. "Subpixel" and "image subpixel" are defined herein to mean a single primary color element that is used, along with two other primary color elements, to form a color from the full spectrum in an image pixel.
Examples of color displays which use additive spatial proximity color synthesis are the shadow-mask cathode ray tube (CRT) technology, and the active-matrix addressed color liquid crystal display (AMCLCD) technology, which utilizes a matrix of individually addressable liquid crystal light valves (LCLV) with integral red, green, and blue color filters for color image formation. Additive spatial proximity color synthesis requires high subpixel density (resolution) because the projected angular subtense of the primary color elements must be encompassed within the spatial integration zones of the human visual system in order for the eye to integrate a set of individual primary color subpixels into the single mixture color to be displayed in the image pixel.
From the perspective of the display's hardware, each primary color subpixel of an image pixel must be individually controllable for color, generally along some quantized range from a minimum of no light to the maximum light the display is capable of producing. Thus, a primary color image subpixel is the equivalent of, and will be referred to herein as a "display pixel". The hardware requirement in additive spatial proximity color synthesis for three "populations" of spatially separated primary color display pixels results in a reduction of available image sampling resolution of a display device of a given size, which, in turn, results in decreased image resolution. In addition, significant losses in display luminance and perceived brightness result from the fact that each of the three primary colors, without regard to its individual contribution to overall luminance or perceived brightness, generally occupies an equal amount of the available, active light emitting surface area of the display. For the display of large full color images in particular, additive spatial juxtaposition color synthesis alone is not an efficient method for generating full color images because of the excessive cost associated with the increased size of the display.
Another approach to color display systems uses subtractive color synthesis. In subtractive display systems, white (full spectrum) light is passed through successive layers of complimentary color filters, each layer being electrically controlled for absorbing a well-defined region of the spectrum. Examples of subtractive color filter technology suitable for direct view and projection color applications may be found in U.S. Pat. No. 5,032,007, issued to Silverstein, et. al., and in U.S. Pat. No. 4,917,465, issued to Conner at. al. While image resolution is generally of high quality as a result of full color control at the individual display pixel level, display quality may be affected by observer viewing angle, a phenomenon resulting from the parallax between the color filter layers. In addition, the use of subtractive, light absorbing techniques for producing colors makes it difficult to achieve sufficiently high overall luminance and perceived brightness, thus reducing overall display brightness. Moreover, the precise control of color rendition in subtractive display systems is complex because of the correlations existing between the color purity (i.e., saturation) and luminance components of colors generated by sets of subtractive color filters.
Still another method for generating full color images is based on additive spatial superposition in which a full color image is produced by the spatial registration of separate images, each comprised of typically one primary color, and optically fused into one full color image for viewing by the observer. Such a system is generally implemented using either CRT or LCLV technology, and is the predominant method used in color projection displays. An example of a color display utilizing additive spatial superposition is disclosed by Dolgoff in U.S. Pat. No. 5,012,274, in FIGS. 1 and 2, and at column 10; and in Jacobson et. al., U.S. Pat. No. 4,127,322. Typically, three images corresponding to red, green, and blue primary colors are generated, requiring three separate imaging (optical) paths, although multiple path systems with more than three optical paths are known in the art, as shown, for example, in the above-mentioned Jacobson, et. al. patent. Because each display pixel is equivalent to an image pixel and is capable of full color and luminance control, and because each of the color images is generated at full spatial resolution, the additive spatial superposition method of color synthesis achieves excellent image resolution and can also achieve relatively high overall luminance and perceived brightness. For these reasons, spatial superposition of separate color images provides the most feasible color synthesis method for producing large full color display images, such as those required in high definition television or comparable visual information display systems.
Both CRT-based and LCLV-based multiple (three or more) optical path, full color, spatial superposition display systems require precisely controlled hardware and optical elements to achieve exact image registration and alignment in order to maintain color purity and image sharpness. In addition, color display systems using projected superimposed images tend to be large, complex, and costly as a result of the separate optical paths and sets of imaging elements needed.
In particular, however, CRT-based full color spatial superposition display systems are deficient for a variety of reasons, but especially because CRT technology has inherent luminance limitations which typically result in dim images which must be viewed in a dimly lit environment, often on a special screen. Improving the brightness of CRT displays often results in lower resolution displays which are not suitable for the high information content required in many display applications.
Liquid crystal technology, then, provides the most flexible and feasible technology for implementing display systems for displaying full color, high resolution, high quality images of arbitrary size. A full color LCLV display utilizing spatial superposition for color synthesis must efficiently achieve enough brightness to display high quality images of varying sizes in a wide range of ambient lighting conditions, and must exhibit high resolution and an enhanced color gamut for high quality imagery to be displayed. Further, however, such a full color LCLV display device must overcome the problems of optical complexity and image alignment generally associated with devices utilizing spatial superposition color synthesis.
A full color display device must be capable of generating image information in three separate spectral bands which are approximately coincident with the spectral sensitivity functions of the three classes of color sensitive human visual photoreceptors. These human spectral sensitivity functions are broadly overlapping, but are generally described as having peak sensitivities in the short, medium, and long visual wavelength bands which roughly correspond respectively to blue, green, and red. It is generally known that the human visual system has a low degree of spatial sensitivity for blue light as compared to the other primary colors. See, for example, Glenn, et. al, Imaging System Design Based on Psychophysical Data, Proceedings of the SID, Volume 26 (1985) pp. 71-78, discussing the characteristics of human spatial sensitivity to different wavelength light energy. The eye's peak spatial response to blue light occurs at approximately one half the spatial frequency of peak spatial response for red or green light and half again the spatial frequency for achromatic, or luminance, signals, indicating that blue light contributes only a minor amount to image resolution factors such as image shape and spatial detail. As a result, neither the resolution nor the alignment of blue image pixels in an image created by additive spatial superposition is critical to image quality since misalignment is not easily detected by the eye. In color display systems using additive spatial juxtaposition color synthesis, the presence of blue display pixels in a proportion equal to red and green display pixels on the display surface of a LCLV color display may actually degrade overall image sharpness, since blue pixels contribute such a minor amount to image resolution.
At the same time, the human visual system's higher sensitivity to spatial resolution in red and green light than in blue light also means that the red and green display pixels must be at a high enough resolution in additive juxtaposition systems to integrate into one color, and that red and green images must be carefully aligned in three-path additive spatial superposition systems so that the eye will not be able to detect the separate red and green images at the image edges.
It is also known that the photopic response of the human eye to blue light is low and inefficient, and thus, blue light provides a much smaller contribution to overall perceived brightness than light from the red and green portions of the visible spectrum. See, for example, Wyszecki and Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, John Wiley & Sons, New York, (2nd Ed. 1982), Section 4.3.2, pp. 256-259, discussing the chromatic and luminous efficiency characteristics of human sensitivity to different wavelength light energy. Thus, blue display pixels contribute little to the total luminance or perceived brightness of a LCLV color display, and, in fact, may actually decrease the potential luminance or overall perceived brightness of the LCLV color display, if represented in equal spatial proportions to the red and green subpixels. Thus, color display devices utilizing additive spatial methods for producing color which allocate equivalent spatial sampling density to the short wavelength (blue) primary display pixel sacrifice a potentially higher spatial resolution of the device, which is rendered solely by middle (green) and long (red) wavelength components, while at the same time limiting the effective brightness and color performance of the device by unnecessary high density sampling of blue display pixels.
However, while blue light contributes little to spatial resolution or brightness, it is fundamental that blue light cannot be ignored if the color display is to produce a full color image, since blue light contributes a disproportionately large amount to the perceived hue and saturation of colors, and can often be the limiting factor in achieving maximum luminance in full color displays. Thus, the highest possible luminance contribution of blue light improves the display's maximum luminous output and overall color balance or white point, and achieves a larger and more balanced color gamut. The present invention is premised on the discovery that the differential treatment of short wavelength (blue) light in LCLV color display technology affords an opportunity for improving color image resolution, enlarging the available color gamut, and improving the overall perceived brightness of a LCLV full color display, while at the same time reducing the complexity and image alignment problems associated with existing full color devices using spatial superposition color synthesis for fusing primary color images created in three optical paths.
The present invention recognizes that increasing the overall space-average intensity of the short wavelength light contribution by reducing the resolution of the blue light image increases the overall intensity contribution of the blue light to the final display image without reducing the effective image resolution. Space-average intensity in this context is defined as the average measured intensity of the light energy collectively emitted from individual display pixels over some small defined display area having a certain pixel resolution. The human visual receptors integrate the individual pixel light energy over the defined area, regardless of how the light is actually distributed. The larger the active light emitting display surface within this area, the brighter the area appears, since, for small areas of the color display under discussion, the perceived brightness is proportional to intensity or luminance. A certain portion of each LCLV pixel's area is devoted to fixed-size, opaque hardware overhead mechanisms needed to address each pixel and store voltage representative of display information. The space-averaged intensity measured for a given display size is thus a function of the remaining, nonstructural, pixel area which emits light. It follows, then, that increasing the area in which light is emitted will increase the space-average intensity of the blue light being emitted and perceived by the eye. Enlarging the display pixels, and consequently reducing the image resolution, means that each pixel is more transparent and has less overall pixel area taken up by the structural overhead mechanisms, thereby increasing the overall area available for emitting blue light and increasing space-average intensity for the blue image. In addition, this increased luminance of the blue image can be achieved efficiently without increasing the intensity of the light source, thus saving power and limiting heat production.
Increasing blue light intensity is also important in achieving a balanced white color, or white point, for the display. The brightest white point that a color display can achieve is limited by the amount of luminance (intensity) that can be achieved for the short wavelength light. That is, because of the eye's low luminous efficiency for short wavelength light energy, the luminance of the display's white point is, in effect, limited by the proportional luminance contribution of blue light to the white mixture color. Thus, increasing luminance for the short wavelength light provides a brighter white point as well. In addition, an increase in blue light intensity will increase the luminance range of all other colors which have a blue component. The present invention uses a separate optical path for providing a separate, brighter blue light image.
Attempts have been made to produce full color images by differentially treating light according to two separate wavelength bands. U.S. Pat. No. 4,886,343, issued to Johnson, discloses a liquid crystal display (LCD) unit in which a first (top) panel (image plane) of display pixel elements is used to control red and green portions of a displayed image using an additive spatial proximity technique, while a second (bottom) panel (image plane), having display pixels aligned with the first panel display pixels, controls the blue portion of the image through a subtractive superpositional technique. Overall perceived brightness of an image displayed on the Johnson device will be diminished by the absorption of light through the panels as a result of using the magenta and cyan filters, and as a result of using the subtractive light absorption method for controlling blue light. Moreover, as shown by FIG. 12 in Johnson, while the device utilizes light in three primary color wavelength bands, the color gamut achievable by the device is nonetheless severely restricted, since the display is incapable of producing colors in the blue region of the chromaticity diagram.
Conner et. al., in U.S. Pat. No. 4,917,465, disclose a display system comprised of stacking three supertwisted nematic (STN) birefringent LCD panels that are tuned to different subtractive primary colors (i.e., yellow, cyan, and magenta), with polarizers interposed between and sandwiched about the stacked panels. At columns 15 and 16, and in FIGS. 32 and 33, Conner et. al. disclose display systems using the stacked STN birefringent LCD panels and their associated polarizers and using split (two) optical paths. While STN technology may permit the liquid crystal cells to be multiplexed at the high rates necessary for high information content displays, their birefringent operating mode results in slow switching times and generally poor color performance. STN cells are unavoidably colored, generally incapable of producing black and white for high contrast, and capable of producing only a small spectrum of selectable colors without additional complex and light-absorbing optical elements such as polarizers to compensate for their poor color performance and contrast. The display system disclosed by Conner et. al., which uses polarizers to compensate for the STN cells, is likely to have limited luminous efficiency and poor contrast. In addition, the display system requires additional complex optical components to achieve optimal gray scale capability.
The present invention for producing full color images is also distinguishable from liquid crystal based color display devices which produce color images using light separated into two primary colors which follow two optical paths. Examples of such two path, two primary color devices include U.S. Pat. No. 4,345,258 and U.S. Pat. No. 4,983,032. These devices cannot produce full color images.